Fix TreeMap's transformTerm wrt UnApply

[anatoliykmetyuk]

UnApply <: Term however it cannot appear where
a term can. Hence we do not transform it from
transformTerm and guard against it getting into
transformTerm to prevent MatchErrors.

[nicolasstucki]

This isn't correct, if tree is a Term then IsTerm.unapply(tree) should never be empty. Where did that tree come from?

[anatoliykmetyuk]

The reason for this is (at least) the UnApply type. Technically it is <: Term but for the purposes of Tasty reflect we do not treat it as such since it cannot appear in a term position.

[anatoliykmetyuk]

We discussed earlier the failing example that this PR fixes:

Macro:

import scala.quoted.{ given, _ }

inline def mcr(x: => Unit): Unit = ${mcrImpl('x)}
def mcrImpl(x: Expr[Unit])(given ctx: QuoteContext): Expr[Unit] =
  import ctx.tasty.{ given, _ }
  val tr: Term = x.unseal
  object m extends TreeMap
  m.transformTerm(tr).seal.cast[Unit]

Test:

@main def Test = mcr {
  val (a, b) = ???
}

[anatoliykmetyuk]

@nicolasstucki the problem seems to be with this method: https://github.com/lampepfl/dotty/blob/04a68b8df2c8f0bfe2f6fc41a7722b63a6dc4098/library/src-bootstrapped/scala/tasty/reflect/TreeUtils.scala#L113

val (a, b) = ??? is correctly represented in Tasty:

Representation

Inlined(
  EmptyTree,List(

  ),Block(
    List(
      ValDef(
        $1$,TypeTree[AppliedType(
          TypeRef(
            ThisType(
              TypeRef(
                NoPrefix,module class scala
              )
            ),class Tuple2
          ),List(
            TypeRef(
              ThisType(
                TypeRef(
                  NoPrefix,module class scala
                )
              ),class Nothing
            ), TypeRef(
              ThisType(
                TypeRef(
                  NoPrefix,module class scala
                )
              ),class Nothing
            )
          )
        )],Match(
          Typed(
            Ident(
              ???
            ),TypeTree[AnnotatedType(
              TypeRef(
                ThisType(
                  TypeRef(
                    NoPrefix,module class scala
                  )
                ),class Nothing
              ),ConcreteAnnotation(
                New(
                  TypeTree[TypeRef(
                    ThisType(
                      TypeRef(
                        NoPrefix,module class scala
                      )
                    ),class unchecked
                  )]
                )
              )
            )]
          ),List(
            CaseDef(
              Typed(
                UnApply(
                  TypeApply(
                    Select(
                      Ident(
                        Tuple2
                      ),unapply
                    ),List(
                      TypeTree[TypeRef(
                        ThisType(
                          TypeRef(
                            NoPrefix,module class scala
                          )
                        ),class Nothing
                      )], TypeTree[TypeRef(
                        ThisType(
                          TypeRef(
                            NoPrefix,module class scala
                          )
                        ),class Nothing
                      )]
                    )
                  ),List(

                  ),List(
                    Bind(
                      a,Ident(
                        _
                      )
                    ), Bind(
                      b,Ident(
                        _
                      )
                    )
                  )
                ),TypeTree[TypeRef(
                  ThisType(
                    TypeRef(
                      NoPrefix,module class scala
                    )
                  ),class Nothing
                )]
              ),EmptyTree,Apply(
                TypeApply(
                  Select(
                    Ident(
                      Tuple2
                    ),apply
                  ),List(
                    TypeTree[TypeRef(
                      ThisType(
                        TypeRef(
                          NoPrefix,module class scala
                        )
                      ),class Nothing
                    )], TypeTree[TypeRef(
                      ThisType(
                        TypeRef(
                          NoPrefix,module class scala
                        )
                      ),class Nothing
                    )]
                  )
                ),List(
                  Ident(
                    a
                  ), Ident(
                    b
                  )
                )
              )
            )
          )
        )
      ), ValDef(
        a,TypeTree[TypeRef(
          ThisType(
            TypeRef(
              NoPrefix,module class scala
            )
          ),class Nothing
        )],Select(
          Ident(
            $1$
          ),_1
        )
      ), ValDef(
        b,TypeTree[TypeRef(
          ThisType(
            TypeRef(
              NoPrefix,module class scala
            )
          ),class Nothing
        )],Select(
          Ident(
            $1$
          ),_2
        )
      )
    ),Literal(
      Constant(
        (

        )
      )
    )
  )
)

But transformTree method does not take the correct case at Typed(Unapply). This is because such a node passes the IsStatement check which goes before the IsUnapply check.